Just a couple notes to add on the last post. I was there at the same time.
1: There was a level 9 cleric, a level 10 fighter type, and a level 13 monk there too, so by no means were you the lowest level.
2: That was a crazy high spawn
3: We likely would have been ok were it not for the scouting. The biggest problem we had was communication. Many wanted to be the hero, but there is no room for heroes, just teamwork in a place like that.
In my experiences there, invisibility is nearly worthless. The pit fiends and other demons dispel everything anyway. They are very hard to hit, I needed 20's to hit them as well.
Glad to see a dungeon we can't just know we will finish everytime, but..... Here's the party last night: level 19? wizard, level 18 fighter type, level 17 druid, level ?? (high) cleric, level 13 cleric, level 13 monk, level 10 fighter type, level 9 cleric. Guess what, everyone working together, still ridiculously hard. We killed about a dozen pit fiends and several other things around, then the server reset. After all that, got a "fairly eventful" on my rest. We spent lots of time, potions, etc, and got basically nothing, but, only 3 or 4 deaths. Tough dungeon.